Objective This paper explores the vertical distribution of fine root biomass (≤ 2 mm) and fine root form of P. tabuliformis plantations under different stand densities, slope and slope direction, so as to provide a basis for vegetation management and ecological construction evaluation in the loess region of western Shanxi Province of northern China.

Method We investigated the vertical distribution and influencing factors of fine root characteristics, including fine root biomass density, length density, tissue density, specific root length and specific surface area in 0−100 cm soil layer of P. tabuliformis under five stand densities M1(< 2 000 tree/ha), M2(2 000−3 000 tree/ha), M3(3 000−4 000 tree/ha), M4(4 000−5 000 tree/ha), M5(> 5 000 tree/ha), three gradients (≤ 20°, 20°−30°, 30°−40°) and two slope aspects (shady slope and sunny slope).

Result (1) The fine root biomass density in the 0−100 cm soil layer exhibited a unimodal pattern with the increase in stand density, reaching its peak at 606.19 g/m³ with M4 density. The slope gradient and aspect showed no significant impact on fine root biomass density. The density of fine root biomass decreased significantly with the increase of soil depth (P < 0.05), mainly concentrated in 0−20 cm soil layer, accounting for more than 38%. (2) With the increase of stand density, the fine root length density of 0−100 cm soil layer showed a unimodal pattern (the maximum value with 3 639.73 m/ m² at M4 density ), and the specific root length and specific surface area showed a trend of decreasing first and then increasing (the minimum values with 601 cm/g and 101.09 cm²/g, respectively). While slope gradient and aspect had no significant effects on the morphology and vertical distribution of fine roots. In the vertical direction, the ratio of fine root length density in 0−40 cm soil layer of each stand density was more than 60%. The specific root length and specific surface area decreased significantly with the increase of stand density in 0−20 cm soil layer only. However, under the conditions of M4 and M5 density, the specific root length and specific surface area decreased first and then increased with increasing depth. (3) Correlation analysis showed that stand density and soil depth significantly influenced fine root biomass density and morphology. Fine root biomass density and root length density were significantly positively correlated with stand density, soil total carbon, total nitrogen and organic carbon, significantly negatively correlated with soil depth and C∶N. Specific root length and specific surface area were significantly negatively correlated with stand density and soil organic carbon, and significantly positively correlated with soil depth (P < 0.05).

Conclusion Stand density is the key regulator in growth of fine roots of P. tabuliformis plantations. These findings provide supports for sustainable management in semi-arid regions.